Long-term dideoxynucleoside (ddN) therapy, of AIDS patients, has resulted in tissue and drug-specific toxicities (AZT-bone marrow toxicity; ddC-neuropathy; ddI-pancreatitis). Our own investigations, achieved during current grant period, have led to new findings, e.g. prolonged exposure of H9 cells to AZT resulted in cellular resistance (H9-AZT cells). H9-AZT cells showed increased thymidine kinase (TK) activity, decreased salvage of thymidine (TdR) and AZT, depletion of dTTP and reduced DNA synthesis. We have also observed that: (i) DNA of H9 and H9-AZT cells markedly differed in susceptibility to restriction endonucleases; (ii) metabolism of AZT, TdR and ddC markedly differed in H9, CEM, and L1210 cells; (iii) AZTMP (AZT-5'-monophosphate) effluxed from cells; and (iv) hydroxyurea and acivicin significantly increased cellular accumulation of dideoxy-cytidine. These data have led us to hypothesize that: ddN interact with cellular targets, other than reverse transcriptase, leading to cellular toxicity; toxicities depend on the differences in drug's intercellular metabolism; there may be a relationship between ddN-nucleotide efflux and toxicity; and combination therapy may enhance the activity of ddN antivirals. Our long-term objective is to understand differences in the metabolism and mechanisms of action of ddN in different human cells and exploit the knowledge to selectively optimize their anti-AIDS efficacies. To achieve this objective, we propose to extend our observations and test the hypothesis by: (i) exploring the reasons for increased TK activity and examine if other ddN also lead to cellular resistance; (ii) study mechanisms of AZTMP efflux, its occurrence in HN-1, K-562, and human bone marrow cells and whether other ddN-nucleotides also efflux; (iii) explore basis for drug specific toxicity in human neuronal (HN-1, neuropathy), erythroid (K-562, bone marrow toxicity), and human bone marrow cells; and (iv) examine if the toxicities can be altered by drug combinations. These studies will enhance our understanding of mechanism based actions of antiviral ddN and lead to their selective use.